CT imaging of dogs with perineal hernia reveals large prostates with morphological and spatial abnormalities

Abstract The etiology of canine perineal hernia (PH) remains unclear, although as a disease of older male dogs, it is likely to be hormonal. The role of the prostate in the formation of PH has been questioned; however, prospective and systematic evaluation of prostates in these dogs is absent in the literature. In this prospective case–control study, CT imaging was used to assess prostatic changes in dogs with PH (n = 46) and compare these findings with those of intact age‐matched male dogs (n = 23). Using the OsiriX® DICOM viewer, we measured prostatic volume and correlated it with the size of the dog by using the length of the sixth lumbar vertebra. In addition, we recorded spatial and morphological changes of the prostate, such as heterogenicity, intra‐ and paraprostatic cysts, and mineralizations, as well as prostatic location and rotation. We found that dogs with PH had larger prostates (P < .001) that more often contained cysts (P < .001) and had larger cyst diameters (P = .013) than age‐matched controls. Prostates of PH dogs also contained paraprostatic cysts (17.4%) and focal mineralizations (32.6%), which were absent in the control group. Abnormal rotation and location of the prostate were common in dogs with PH. In conclusion, these findings support the use of CT as an adjunct diagnostic imaging modality for the evaluation of the prostate in dogs with PH. Further studies are needed to evaluate nonprostatic CT findings in the pelvic cavity of PH dogs.

common but relatively unreliable. 8,9 Ultrasound examination, the current gold standard for examining prostatic size and morphology, can be challenging in dogs with PH because of the prostate's location. [7][8][9] Increasing in popularity, CT imaging has been reported to be helpful for evaluating prostatic morphology and size without superimposition of surrounding tissue. 9,10 Computed tomography also allows for correlation of prostatic volume with the size of the dog by using structures such as the length of the sixth lumbar vertebra (L6). 10,11 Prostatic size estimations typically involve single parameters such as height, width, and length; volumetric measurements appear, however, to be more accurate and are becoming increasingly available. [10][11][12] No previously published studies were found assessing the size, morphology and location of the prostate in dogs with PH using CT imaging.
The primary aims of this study were to determine the prostatic volume of dogs with PH using CT imaging and to compare the results with the control group of age-matched intact male dogs. A secondary aim was to describe the morphological and spatial changes of prostates in dogs with PH. Our primary hypothesis was that dogs with PH have larger prostates than their age-matched controls. Our secondary hypothesis was that prostates of dogs with PH are heterogeneous, contain abnormal quantities of cysts and mineralizations, and have abnormal location and rotation.

Study design and selection of dogs
This case-control study consisted of two groups of male dogs, a prospective group suffering from PH (PH group) and a retrospective group gathered from the Veterinary Teaching Hospital of the University of Helsinki (VTHUH) database (control group).
The PH group included male dogs with naturally occurring PH that were referred to the VTHUH from March 2017 to December 2020.
We excluded dogs with previously surgically treated PH, American Society of Anesthesiologists' physical status rating of 4 or 5, severe renal or hepatic disease, or other diseases that might increase the risk for anesthesia. We also excluded dogs that had been surgically or chemically castrated. Decisions for inclusion and exclusion were made by an ECVS-certified veterinary surgeon (S.M.

CT imaging
In the PH group, the dogs underwent a physical examination and hematological and biochemical blood value analyses to assess health status. Clinical assessments were performed by a PhD researcher in clinical veterinary medicine (T.Å.

Volumetric measurements
The prostate perimeters were manually traced in original transverse images with the "Closed Polygon" tracing tool in each slice that contained prostatic tissue, after which the volume was measured using the "Compute Volume" tool. The urethra as well as any cysts connected to the prostate were included in the volume. Volumetric measurements were performed once. The length of L6 was measured at the shortest part of the vertebra in sagittal multiplanar reformatting (MPR). To correlate prostatic volume with the size of the dog, we calculated the ratio volume (RV) by the following formula: cubic root of the prostatic volume divided by L6 length. 12 A 95% confidence interval (CI) for prostatic RV in the control group was used to differentiate between a small, medium, and large prostate. We defined prostates as small if they were below the lower CI and large if they exceeded the upper CI. These were used as reference values in the PH group.

Morphological and spatial assessments
Without histological samples, the description of prostatic changes was based on CT images alone. The parenchymal appearance was described as either homogeneous or heterogeneous. As cysts and abscesses appear similar on CT images, focal rounded or irregular hypoatten- The position of the prostate was defined as abdominal if over 80% of the prostate's length was in front of the cranial border or in the hernia if over 80% was located behind the caudal border. Prostates that did not meet these criteria were described as pelvic.
Rotation of the prostate was described as deviation of the prostatic axis from the midline of the dog, shown in Figure 2A  The location of the bladder, unlike the prostate, cannot be defined by the location of its body, as its size varies depending on its grade of filling. In our study, bladder location was described as abdominal, pelvic, or located in the hernia, depending on the location of the bladder neck. 13 Possible S-shaped cranio-caudal deviation of the urethra occurring caudal to the ischial arch was recorded (no/yes) ( Figure 1). were considered statistically significant.

Selected dogs
In

Volumetric measurements
The mean ± SD prostatic volume was 56.0 cm 3 ± 36.6 (range 13.0-145.6) and 31.6 cm 3 ± 33.2 (range 1.5-125.9) in the PH and control groups, respectively. The mean ± SD RV was 1.66 ± 0.31 (range 1.08-2.76) and 1.23 ± 0.24 (range 0.78-1.68) in the PH and control groups, respectively. The RV was significantly (P < .001) larger in the PH group than in the control group ( Figure 3)

Morphological and spatial assessments
Heterogenicity of the prostate in CT images was evident in 97.8% (45/46) and 52.2% (12/23) of the PH and control groups, respectively.
The difference between groups was significant (P < .001). In the PH group, the prevalence of intraprostatic cysts was 93.5%  transverse planes, respectively. The difference between groups was significant in both dorsal (P = .003) and transverse (P < .001) planes.
In both groups, prostates located in the abdomen seemed to be least   Figure 1D).

DISCUSSION
Based on the authors' review of the literature, this was the first published prospective study to investigate prostatic changes associated with PH using CT imaging. The findings supported both of our hypotheses. First, we found that dogs with PH had significantly larger prostates than control dogs. Second, prostates in the PH group were more often heterogeneous, contained cysts, paraprostatic cysts, and focal mineralizations, and had abnormal rotation and location. In addition, the cranio-caudal flexure of the urethra, a common finding in the PH group, was absent in the control group.
The incidence of prostatic disease is reported to be 10-80% in dogs with PH, with the diagnosis usually based on rectal palpation, radiography, ultrasound, or a combination of these. 4,6,7,[13][14][15] One study questioned the role of prostatic disease in PH formation, as only 5 of 43 dogs had a significantly enlarged prostate on radiographs. 4 Prostates were deemed enlarged if the ratio of prostatic diameter to the pubis-tosacral promontory measurement was >70. In our study, most prostates were displaced caudally, making radiographic assessment challenging due to tissue superimposition. CT imaging avoids this, providing more detail regarding both size and structure. [8][9][10] In addition, we measured volume instead of one-dimensional parameters and found the prostates in the PH group to be enlarged. A study with a similar incidence of prostatic enlargement to ours is that by Merchav et al., who prospectively investigated relaxin receptors in the perineal muscles of dogs with PH. They noted that 80% (12/15) of dogs had prostatic enlargement; however, they did not elaborate on how this diagnosis was made. 13 There is a limited number of studies assessing canine prostatic volume on CT images, this being the first to do so in dogs with PH.

Benign prostatic hyperplasia (BPH) is common in intact male dogs and
is thought to be the underlying cause of most prostatic diseases. 8  The presence of intraprostatic or paraprostatic cysts and mineralization in the prostate of the dog implies a more severe pathologic process than simple enlargement. 8,9,17 Prostatic cysts are the accumulation of prostatic fluid caused by obstruction of canaliculi, often associated with BPH. These cysts are initially microscopic, but as the pathology escalates, the cysts connect, forming macroscopic cysts of various shapes and sizes. 17 Paraprostatic cysts have been described as large, thin-walled cysts attached to the prostate or have been associated with remnants of embryonic tissue or protruding prostatic retention cysts. They are considered to be rare, although the prevalence is unknown. 17,18 Prostate mineralizations are typically small, focal lesions that can be associated with prostatic disease, especially prostatic neoplasia. 19,20 Haverkamp et al. found that 84.8% of dogs aged over 4 years had prostatic cysts in CT images, 12 which is a larger proportion than in our control group (43.5%) and closer to that of our PH group (93.5%).
This finding can partly be explained by the difference in the diameter of the lesion defined as a cyst, which in our study was > 5 mm and in the Haverkamp study was only > 1.2 mm. 12 Mantziaras et al. examined the prostates of 1003 male dogs aged 1-18 years with ultrasound and found prostatic cysts in 33.6%, paraprostatic cysts in 1.6%, and mineralization in 7% of intact male dogs. 21 These findings, considering the age distribution, are similar to our control group.
In dogs with PH, prostatic morphology has been assessed even less than size, 7,15 as palpation, visualization, and radiography are unreliable methods for intraprostatic assessment. 9  In the PH group, focal prostatic mineralizations were common (32.6%). The disease process underlying prostatic mineralization in intact male dogs is unclear, although mineral accumulations in the walls of prostatic cysts are suspected. An association has been drawn between neoplasia and mineralization in the prostates of castrated dogs; however, in intact male dogs, prostatic cancer is uncommon. 19,20 In dogs with PH, herniation of the prostate is relatively common, occurring in 4-40% of PH cases, 4,6,7 which is consistent with our finding of 28.3%. In dogs, the location of the prostate typically changes with age, migrating from an intrapelvic to abdominal site as the prostate grows. 8,20 Although the dogs of our PH group had enlarged prostates, only 6.5% of prostates were in the abdominal cavity, while 65.2% were in the pelvic cavity. We suggest that this is due to increased pressure from the abdominal cavity as the dog struggles to defecate, pushing the prostate caudally into the pelvic cavity and hernia. An S-shaped flexure of the urethra was found in 60.9% of PH dogs, an anomaly that, to the authors' knowledge, has not been described in previous literature on PH. Although the incidence of urethral flexures in male dogs is thought to be uncommon, in bitches, it is associated with the intrapelvic location of the bladder. 23,24 In healthy dogs, the pelvic location of the bladder neck can be an incidental finding, although it has been associated with urinary incontinence. 25 Urinary obstruction caused by a flexure of the urethra has been reported in three male Yorkshire terriers with pelvic bladders. 23,24 We suggest that the relatively high incidence of urethral flexures in dogs with PH is due to the caudal displacement of the prostate toward or into the hernia.
The hormone relaxin has been suggested to play a role in the etiopathogenesis of PH. 5,13 In male dogs, relaxin is mainly of prostatic origin, with relaxin immunoreactivity markedly increased in prostatic and paraprostatic tissue of dogs with PH. 5 In addition, significantly higher expression of relaxin receptors occurs in the perineal muscles of dogs with PH than in healthy intact dogs. 13 It has been proposed that relaxin leaks from a cystic prostate into the surrounding tissues, leading to perineal muscle atrophy. 5  prostatic tissue may be difficult to discern from surrounding tissue, such as the rectum and adjacent muscles. To avoid the risk of volume overestimation, we were careful to exclude all tissue and cysts not obviously connected to the prostates. Heterogenicity and small cysts are easier to detect when using contrast medium; the lack of a contrast agent in the control group may have affected our results. We chose to include cysts that were >5 mm in size to minimize this risk, as smaller cysts can be more easily overlooked. Paraprostatic cysts, mineralizations, and rotation and location of the prostate should not be significantly affected by use of a contrast agent. Additionally, the formerly mentioned unpublished data show that the repeatability of volumetric measurements is high, which is why volumetric measurements were performed only once.
In conclusion, these findings support the use of CT as an adjunct imaging modality for the evaluation of prostatic changes in PH dogs.
In dogs with PH, prostatic changes on CT images significantly differed from changes in control dogs. These findings can be used to justify recommending castration in conjunction with herniorrhaphy for dogs with PH. Further studies are needed to evaluate nonprostatic CT findings in the pelvic cavity of PH dogs.

ACKNOWLEDGMENTS
The authors would like to acknowledge Jouni JT Junnila for assisting with the statistical analysis.